Zoom optical system

ABSTRACT

A zoom optical system includes three lens groups, arranged as follows from the object side: a first lens group having positive refractive power, a second lens group having negative refractive power, and a third lens group having positive refractive power. The first lens group includes, arranged as follows along the optical axis, a lens element having negative refractive power, a prism for bending the optical axis, and a lens element having positive refractive power. The second and third lens groups are movable along the optical axis for zooming with only the third lens group reversing its direction of movement along the optical axis during zooming from the wide-angle end to the telephoto end. The third lens group includes a stop, a cemented lens component, and at least one aspheric surface that may be made of plastic. The first lens group also includes at least one aspheric surface.

FIELD OF THE INVENTION

The present invention relates to a zoom optical system used in an imagepickup device such as a portable telephone, a portable computer, or asimilar device.

BACKGROUND OF THE INVENTION

Recently, in devices such as portable telephones and portable computers,picture image information is incorporated in an image pickup device. Inan image recording or photographic optical system used in such an imagepickup device, it is necessary to make the optical system light-weightand compact in order to obtain the desired portability. When the opticalaxis of an objective lens extends in the thickness direction of an imagepickup device, a technique for reducing the thickness of the casing ofthe device has been developed that uses a prism to bend the optical axisof the optical system.

On the other hand, additional functionality is also desirable in imagerecording and photographic optical systems used in such image pickupdevices, and image recording and photographic optical systems thatinclude a zoom function have been proposed, for example, in JapaneseLaid-Open Patent Application 2004-264585 and Japanese Laid-Open PatentApplication 2000-131610.

However, in Japanese Laid-Open Patent Application 2004-264585, in thecase of forming the zoom lens of three lens groups With the third lensgroup from the object side being movable both for zooming and forfocusing, the third lens group is moved continuously toward the objectside during zooming toward the telephoto end so that the distancebetween the second lens group and the third lens group becomes small.Thus, little distance is left for assuring sufficient movement of thethird lens group for focusing. Particularly, in short-distancephotography, focusing movement becomes greater at the telephoto end thanat the wide-angle end so that the focusing lens, which in this case isthe third lens group, requires increased movement. That is, largemovements are required at the telephoto end. Japanese Laid-Open PatentApplication 2000-131610 operates differently with a zoom lens thatincludes four lens groups, but the use of the four lens groups limitsminiaturization and reduction of the costs of the zoom lens.

BRIEF SUMMARY OF THE INVENTION

The present invention relates to a zoom optical system that is small,that has an inexpensive construction, that assures sufficient movementof the third lens group from the object side for focusing as desired,and that is capable of short-distance photography at the telephoto end.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description given below and the accompanying drawings, whichare given by way of illustration only and thus are not limitative of thepresent invention, wherein:

FIGS. 1A-1B show the cross-sectional views of the zoom optical system ofEmbodiment 1 at the wide-angle end and at the telephoto end,respectively;

FIGS. 2A-2D show the spherical aberration, astigmatism, distortion, andlateral color, respectively, of the zoom optical system of Embodiment 1at the wide-angle end;

FIGS. 2E-2H show the spherical aberration, astigmatism, distortion, andlateral color, respectively, of the zoom optical system of Embodiment 1at the telephoto end;

FIG. 3A-3B show cross-sectional views of the zoom optical system ofEmbodiment 2 at the wide-angle end and at the telephoto end,respectively;

FIGS. 4A-4D show the spherical aberration, astigmatism, distortion, andlateral color, respectively, of the zoom optical system of Embodiment 2at the wide-angle end; and

FIGS. 4E-4H show the spherical aberration, astigmatism, distortion, andlateral color, respectively, of the zoom optical system of Embodiments 2at the telephoto end.

DETAILED DESCRIPTION OF THE INVENTION

A general description of the zoom optical system of the presentinvention that pertains to disclosed embodiments of the invention willnow be described with reference to FIGS. 1A-1B that show Embodiment 1.In FIG. 1A, the object side of the zoom optical system is at the leftwhere the reference symbol X indicates the optical axis of the zoomoptical system. The image pickup plane, indicated by reference symbol 1in FIG. 1A, is on the image side of the zoom optical system. In FIG. 1A,lens elements are referenced by the letter L with a subscript numberdenoting their order from the object side of the zoom optical systemalong the optical axis X, from L₁ to L₇. Similarly, the radii ofcurvature of the optical surfaces are referenced by the letter R with asubscript number denoting their order from the object side of the zoomoptical system, from R₁ to R₁₈. The on-axis surface spacings along theoptical axis X of the various optical surfaces are referenced by theletter D with a subscript number denoting their order from the objectside of the zoom optical system, from D₁ to D₁₇. In the same manner,three lens groups are labeled G₁, G₂, and G₃ in order from the objectside of the zoom optical system, and the optical components belonging toeach lens group are indicated by brackets adjacent the labels G₁, G₂,and G₃.

The term “lens group” is defined in terms of “lens elements” and “lenscomponents” as explained herein. The term “lens element” is hereindefined as a single transparent mass of refractive material having twoopposed refracting surfaces, which surfaces are positioned at leastgenerally transversely of the optical axis of the zoom optical system.The term “lens component” is herein defined as (a) a single lens elementspaced so far from any adjacent lens element that the spacing cannot beneglected in computing the optical image forming properties of the lenselements or (b) two or more lens elements that have their adjacent lenssurfaces either in full overall contact or overall so close togetherthat the spacings between adjacent lens surfaces of the different lenselements are so small that the spacings can be neglected in computingthe optical image forming properties of the two or more lens elements.Thus, some lens elements may also be lens components. Therefore, theterms “lens element” and “lens component” should not be taken asmutually exclusive terms. In fact, the terms may frequently be used todescribe a single lens element in accordance with part (a) above of thedefinition of a “lens component.” The term “lens group” is hereindefined as an assembly of one or more lens components in optical seriesand with no intervening lens components along an optical axis thatduring zooming is movable as a single unit relative to another lenscomponent or other lens components.

The zoom optical system of the present invention includes, arranged inorder from the object side, a first lens group G₁ having positiverefractive power, a second lens group G₂ having negative refractivepower, and a third lens group G₃ having positive refractive power.

A light beam incident along the optical axis X from the object sidepasses through lens groups G₁, G₂, and G₃ and is imaged on the imagepickup plane 1 where an image pickup element, such as a CCD, is located.Moreover, a cover glass 4, which may also include a filter element, isarranged between the third lens group G₃ and the image pickup plane 1.

While FIG. 1A shows the positions of lens groups G₁, G₂, and G₃ at thewide-angle end, lines adjacent reference symbols G₂ and G₃ indicate thelocus of points of movement of lens groups G₂ and G₃ along the opticalaxis X during zooming from the wide-angle end to the telephoto end, withFIG. 1B showing the positions of the lens groups at the telephoto end.The straight line adjacent reference symbol G₂ indicates that lens groupG₂ moves continuously toward the image side during zooming from thewide-angle end to the telephoto end. The line adjacent reference symbolG₃ is convex upward, that is, convex toward the object side, andindicates that the third lens group G₃ moves first toward the objectside and then back toward the image side during zooming from thewide-angle end to the telephoto end. The second lens group G₂ and thethird lens group G₃ generally become closer together during zooming fromthe wide-angle end to the telephoto end, and the movements enable thethird lens group G₃ having positive refractive power to move properlyalso for focusing adjustment, simplifying the construction and movementsrequired so that costs can be reduced and particularly enablingshort-distance photography at the telephoto end.

As shown in FIG. 1A, the first lens group G₁ includes, arranged in orderfrom the object side, a first lens element L₁ having negative refractivepower, a prism 2 for bending the optical axis, and a second lens elementL₂ having positive refractive power. Miniaturization and cost reductioncan be achieved by arranging at least one lens element having negativerefractive power on the object side of the prism 2 so as to decrease thediameter of the light beam to the prism 2 and thus reduce the requiredsize of the prism 2. It is preferable that a lens element havingpositive refractive power of the first lens group G₁ include an asphericsurface, and when an aspheric lens element is used as the second lenselement L₂, field curvature and distortion can be well corrected.

The second lens group G₂ includes, arranged in order from the objectside, a biconcave third lens element L₃ and a fourth lens element L₄having positive refractive power.

The third lens group G₃ includes, arranged in order from the objectside, a stop 3, a lens component that includes a fifth lens element L₅having positive refractive power that is cemented on its image side to asixth lens element L₆ having negative refractive power, and another lenscomponent that is a lens element L₇. It is preferable that the thirdlens group G₃ include a stop, shown as stop 3 in FIG. 1A, forcontrolling the amount of light that passes through the zoom opticalsystem. The axial chromic aberration and the lateral color can becorrected by such a construction, and the spherical aberration and fieldcurvature can be corrected by having the separate lens element L₇include at least one aspheric surface. Moreover, because it is easy toform the aspheric lens element, lens element L₇, and the cemented lenselements L₅ and L₆ so that their opposed surfaces properly connect byforming the aspheric lens element of plastic, the suppression ofeccentricity within the third lens group G₃ and the stabilization ofimage quality of the entire zoom optical system can thus be achieved.

The lens surface or surfaces that are aspheric are defined using thefollowing equation:Z=[(Y ² /R)/{1+(1−K·Y ² /R ²)^(1/2)}]+Σ(A _(i) ·Y ^(i))  Equation (A)where

-   -   Z is the length (in mm) of a line drawn from a point on the        aspheric lens surface at a distance Y from the optical axis to        the tangential plane of the aspheric surface vertex,    -   R is the radius of curvature (in mm) of the aspheric lens        surface on the optical axis,    -   Y is the distance (in mm) from the optical axis,    -   K is the eccentricity, and    -   A_(i) is the ith aspheric coefficient, and the summation extends        over i.

In embodiments of the invention disclosed below, only asphericcoefficients A₃-A₁₂ are non-zero.

It is preferable that the zoom optical system of the present inventionsatisfies the following Condition (1):0<(Lm−Lt)/Fw<0.2  Condition (1)where

-   Lm is the distance along the optical axis X from the vertex of the    most object-side lens surface of the third lens group G₃ to the    image surface when the third lens group G₃ is nearest the object    side during zooming;-   Lt is the distance along the optical axis X from the vertex of the    most object-side lens surface of the third lens group G₃ to the    image surface at the telephoto end of the zoom range; and-   Fw is the focal length of the zoom optical system at the wide-angle    end of the zoom range.

Condition (1) above relates to the movement of the third lens group G₃during zooming, enables short-distance photography at the telephoto end,and further increases the light-receiving efficiency of a CCD imagepickup element by Condition (1) being satisfied. If the upper limit ofCondition (1) is not satisfied, the third lens group G₃ comes too closeto the image side and the exit angle of light rays passing to the CCDincreases, reducing the light-receiving efficiency of the CCD. On theother hand, if the lower limit of Condition (1) is not satisfied, thedistance between the second lens group G₂ and the third lens group G₃becomes too small during zooming, and the amount of movement of thethird lens group G₃ required for focusing cannot be ensured,particularly making short-distance photography difficult.

Two embodiments of the present invention will now be individuallydescribed with reference to the drawings.

Embodiment 1

FIGS. 1A-1B show cross-sectional views of the zoom optical system ofEmbodiment 1 at the wide-angle end and at the telephoto end,respectively. As shown in FIG. 1A, the zoom optical system of Embodiment1 includes, arranged in order from the object side, a first lens groupG₁ having positive refractive power, a second lens group G₂ havingnegative refractive power, and a third lens group G₃ having positiverefractive power. The straight line adjacent reference symbol G₂ and theconvex upward line adjacent reference symbol G₃ taken together indicatethat as the second lens group G₂ moves at a constant speed along theoptical axis X between the wide-angle end shown in FIG. 1A and thetelephoto end shown in FIG. 1B, the third lens group G₃ moves at avarying speed along the optical axis X.

In Embodiment 1, the first lens group G₁ includes, arranged in orderfrom the object side, a first lens element L₁ having negative refractivepower, a meniscus shape, and a convex surface on the object side, aprism for bending the optical axis, and a second lens element L₂ havinga biconvex shape with two aspheric surfaces.

The second lens group G₂ includes, arranged in order from the objectside, a third lens element L₃ having a biconcave shape with two asphericsurfaces and a fourth lens element L₄ having positive refractive power,a meniscus shape, and a convex surface on the object side.

The third lens group G₃ includes, arranged in order from the objectside, a fifth lens element L₅ having a biconvex shape, a sixth lenselement L₆ having a biconcave shape, and a seventh lens element L₇having a biconvex shape with two aspheric surfaces.

Table 1 below lists the surface number # (the stop 3 defining theeleventh surface), in order from the object side, the radius ofcurvature R (in mm) of each surface on the optical axis, the on-axissurface spacing D (in mm) except that the on-axis surface spacings thatvary with zooming are listed in Table 3 below, as well as the refractiveindex N_(d) and the Abbe number v_(d) at the d-line (587.6 nm) of eachoptical element for Embodiment 1. Note that although R is the on-axisradius of curvature, for convenience of illustration, in FIG. 1A thelead lines from the R reference symbols extend to the surfaces beingreferenced but do not extend to the on-axis positions. Listed in thebottom portion of Table 1 are the focal length f and the f-number F_(NO)at the wide-angle and telephoto ends, and the maximum field angle 2ω atthe wide-angle end and at the telephoto end for Embodiment 1. Tablessimilar to those for Embodiment 1 below will be used later to describeEmbodiment 2 of the present invention. TABLE 1 # R D N_(d) ν_(d)  140.7061 0.75 1.80518 25.4  2 11.0003 2.73  3 ∞ 8.90 1.78590 44.2  4 ∞0.05  5* 15.5556 2.51 1.58809 60.4  6* −17.4293 D₆(variable)  7*−20.0619 0.80 1.80348 40.4  8* 6.2807 1.06  9 9.0982 1.70 1.92286 18.910 25.2351 D₁₀(variable) 11(stop) ∞ 0.50 12 5.8040 3.71 1.84666 23.8 13−5.8040 0.80 1.92286 18.9 14 4.9013 0.20 15* 5.4510 2.30 1.51007 56.216* −29.9187 D₁₆(variable) 17 ∞ 0.85 1.51680 64.2 18 ∞ f = 6.67-18.85F_(NO) = 4.53-5.10 2ω = 60.4°-21.4°The lens surfaces with a * to the right of the surface number in Table 1are aspheric lens surfaces.

Table 2 below lists the values of the constant K and the coefficientsA₃-A₁₂ used in Equation (A) above for each of the aspheric lens surfacesof Table 1. Aspheric coefficients that are not present in Table 2 arezero. An “E” in the data indicates that the number following the “E” isthe exponent to the base 10. For example, “1.0E-2” represents the number1.0×10⁻². TABLE 2 # K A₃ A₄ A₅ A₆  5 1.4912069 3.9208178E−5 2.7667705E−5−4.9563963E−5 7.1616506E−6  6 −4.5561462 1.6307361E−4 −9.9910950E−5−1.6903019E−5 −8.0880629E−7  7 −97.5840668 8.5228393E−4 −1.2350851E−31.3308189E−4 −1.8120603E−5  8 1.4976842 4.5258735E−4 5.1896427E−4−5.5754680E−4 5.0146003E−5 15 5.0948807 −5.9417278E−4 −1.6108152E−3−2.9546806E−3 1.0591787E−3 16 −9.9915499 −2.1756999E−4 1.3341972E−36.7729418E−5 −3.5289965E−4 A₇ A₈ A₉ A₁₀ A₁₁ A₁₂  5 −8.0528622E−73.0010886E−8 1.4064101E−8 2.2801412E−10 −1.3713204E−10 −8.6099389E−11  62.2953182E−7 1.3608465E−7 −4.7668751E−9 −2.4602899E−9 −2.9444038E−104.8229839E−12  7 7.2991718E−6 3.7584936E−7 −3.3012359E−7 2.3419032E−8 00  8 −5.2337496E−6 3.7703741E−6 −2.6762316E−7 −9.0003768E−8 0 0 158.2889684E−5 −1.5877049E−4 −8.6534314E−5 −8.2764803E−6 4.9781809E−5−1.5962419E−5 16 8.2617933E−5 8.9294154E−5 8.4040112E−6 −2.1462606E−5−6.5522234E−6 3.7610168E−6

In the zoom optical system of Embodiment 1, lens groups G₂ and G₃ movealong the optical axis during zooming to vary the separations of thethree lens groups in order to provide a zoom ratio of about three.Therefore, the values of the on-axis spacings D₆, D₁₀, and D₁₆ vary.Table 3 below lists the values of the variables D₆, D₁₀, and D₁₆ (i.e.,the group spacings) at the wide-angle end, at the middle position atwhich the third lens group G₃ reaches its object-most position, and atthe telephoto end with the zoom optical system focused at infinity.TABLE 3 Focal Length f (mm) D₆ D₁₀ D₁₆ 6.67 0.35 15.40 12.45 16.47 8.394.64 15.17 18.85 10.26 3.30 14.64

Embodiment 1 satisfies Condition (1) above with a value of (Lm−Lt)/Fw of0.079. The entire length of the zoom optical system is 55.06 mm.

FIGS. 2A-2D show the spherical aberration, astigmatism, distortion, andlateral color, respectively, of the zoom optical system of Embodiment 1at the wide-angle end. FIGS. 2E-2H show the spherical aberration,astigmatism, distortion, and lateral color, respectively, of the zoomoptical system of Embodiment 1 at the telephoto end. In FIGS. 2A and 2E,the spherical aberration (in mm) is shown for the wavelengths 587.6 nm(the d-line), 460 nm, and 615 nm, and the f-number (F/ ) is shown. Inthe remaining figures, ω is the half-field angle. In FIGS. 2B and 2F,the astigmatism (in mm) is shown for both the sagittal image surface S(solid line) and the tangential image surface T (broken line) and ismeasured at 587.6 nm (the d-line). In FIGS. 2C and 2G, distortion (inpercent) is measured at 587.6 nm (the d-line). In FIGS. 2D and 2H, thelateral color (in μm) is shown for the wavelengths 460 nm and 615 nmrelative to 587.6 nm (the d-line).

As is evident from FIGS. 2A-2H and from the numerical data in the tablesabove, aberrations are extremely well corrected in Embodiment 1 of thepresent invention.

Embodiment 2

FIGS. 3A-3B show cross-sectional views of the zoom optical system ofEmbodiment 2 at the wide-angle end and at the telephoto end,respectively. Embodiment 2 is similar to Embodiment 1 and therefore onlythe differences between Embodiment 2 and Embodiment 1 will be explained.Embodiment 2 differs from Embodiment 1 in having a third lens element inthe second lens group G₂, which becomes the fifth lens element L₅ ascounted from the object side of the zoom optical system. Embodiment 2also differs from Embodiment 1 in its lens element configuration byhaving different radii of curvature of the lens surfaces, differentaspheric coefficients of the aspheric lens surfaces, different opticalelement surface spacings, and some different refractive indexes and Abbenumbers.

The second lens group G₂ includes, arranged in order from the objectside, a third lens element L₃ having a biconcave shape and a lenscomponent formed of a fourth lens element L₄ having a biconcave shapethat is cemented to the fifth lens element L₅ that has a biconvex shape.

A main difference of Embodiment 2 from Embodiment 1 is that the secondlens group G₂ includes three lens elements, rather than two, and two ofthe three lens elements are cemented together. Lateral color isparticularly further improved by having a cemented lens component in thesecond lens group G₂.

Table 4 below lists the surface number # (the stop 3 defining thetwelfth lens surface), in order from the object side, the radius ofcurvature R (in mm) of each surface on the optical axis, the on-axissurface spacing D (in mm) except that the on-axis surface spacings thatvary with zooming are listed in Table 6 below, as well as the refractiveindex N_(d) and the Abbe number v_(d) at the d-line (587.6 nm) of eachoptical element for Embodiment 2. Note that although R is the on-axisradius of curvature, for convenience of illustration, in FIG. 3A thelead lines from the R reference symbols extend to the surfaces beingreferenced but do not extend to the on-axis positions. Listed in thebottom portion of Table 4 are the focal length f and the f-number F_(NO)at the wide-angle and telephoto ends, and the maximum field angle 2ω atthe wide-angle end and at the telephoto end for Embodiment 2. TABLE 4 #R D N_(d) ν_(d)  1 30.7982 0.75 1.80518 25.4  2 11.0001 2.90  3 ∞ 8.901.78590 44.2  4 ∞ 0.05  5* 15.0026 2.46 1.58809 60.4  6* −19.2166D₆(variable)  7 −24.3926 0.58 1.83400 37.2  8 8.4442 1.00  9 −23.77830.56 1.54814 45.8 10 8.3462 1.84 1.84666 23.8 11 −58.4469 D₁₁(variable)12(stop) ∞ 0.50 13 6.3023 3.71 1.84666 23.8 14 −10.2514 0.80 1.9228618.9 15 4.8588 0.10 16* 4.3969 2.30 1.51007 56.2 17* −50.7759D₁₇(variable) 18 ∞ 0.85 1.51680 64.2 19 ∞ f = 6.66-18.85 F_(NO) =4.47-5.01 2ω = 60.6°-21.2°The lens surfaces with a * to the right of the surface number in Table 4are aspheric lens surfaces.

Table 5 below lists the values of the constant K and the coefficientsA₃-A₁₂ used in Equation (A) above for each of the aspheric lens surfacesof Table 4. Aspheric coefficients that are not present in Table 5 arezero. An “E” in the data indicates that the number following the “E” isthe exponent to the base 10. For example, “1.0E-2” represents the number1.0×10⁻². TABLE 5 # K A₃ A₄ A₅ A₆  5 2.8205941 −1.4202532E−47.1781566E−5 −8.6863684E−5 1.4145887E−5  6 −1.4760056 7.3141867E−5−1.4716244E−4 2.9762562E−5 1.9800893E−6 16 3.2382971 −2.2404145E−31.7562041E−3 −5.6192401E−3 2.1588192E−3 17 5.1685197 −9.9917874E−42.5854806E−3 4.4154191E−4 −4.0786940E−4 A₇ A₈ A₉ A₁₀ A₁₁ A₁₂  59.1188743E−7 4.6872439E−8 −2.5387916E−8 −1.0398101E−8 −1.1495803E−95.8358231E−10  6 4.0357352E−7 5.8136643E−8 −2.4487058E−8 −5.9287218E−9−4.8049945E−10 4.4091685E−10 16 8.6049096E−6 −2.6483992E−4 −7.9361455E−5−4.8978953E−6 6.6130382E−5 −2.2273502E−5 17 −6.0627627E−5 6.7449820E−55.2708384E−5 5.3740206E−6 −2.4370307E−5 5.8671554E−6

In the zoom optical system of Embodiment 2, lens groups G₂ and G₃ movealong the optical axis during zooming to vary the separations of thethree lens groups in order to provide a zoom ratio of about three.Therefore, the values of the on-axis spacings D₆, D₁₁, and D₁₇ vary.Table 6 below lists the values of the variables D₆, D₁₁, and D₁₇ (i.e.,the group spacings) at the wide-angle end, at the middle position atwhich the third lens group G₃ reaches its most object-side position, andat the telephoto end with the zoom optical system focused at infinity.TABLE 6 Focal Length f (mm) D₆ D₁₁ D₁₇ 6.66 0.35 14.74 12.68 16.26 8.044.46 15.27 18.85 10.07 3.10 14.60

Embodiment 2 satisfies Condition (1) above with a value of (Lm−Lt)/Fw of0.101. The entire length of the zoom optical system is 55.07 mm.

FIGS. 4A-4D show the spherical aberration, astigmatism, distortion, andlateral color, respectively, of the zoom optical system of Embodiment 2at the wide-angle end. FIGS. 4E-4H show the spherical aberration,astigmatism, distortion, and lateral color, respectively, of the zoomoptical system of Embodiment 2 at the telephoto end. In FIGS. 4A and 4E,the spherical aberration (in mm) is shown for the wavelengths 587.6 nm(the d-line), 460 nm, and 615 nm, and the f-number (F / ) is shown. Inthe remaining figures, ω is the half-field angle. In FIGS. 4B and 4F,the astigmatism (in mm) is shown for both the sagittal image surface S(solid line) and the tangential image surface T (broken line) and ismeasured at 587.6 nm (the d-line). In FIGS. 4C and 4G, distortion (inpercent) is measured at 587.6 nm (the d-line). In FIGS. 4D and 4H, thelateral color (in μm) is shown for the wavelengths 460 nm and 615 nmrelative to 587.6 nm (the d-line).

As is evident from FIGS. 4A-4H and from the numerical data in the tablesabove, aberrations, especially lateral color, are well corrected inEmbodiment 2 of the present invention.

The zoom optical system of the present invention being thus described,it will be obvious that the same may be varied in many ways. Forinstance, values such as the radius of curvature R of each of the lenselements, the surface spacing D, the refractive index N_(d), as well asthe Abbe number V_(d), are not limited to the examples indicated in eachof the aforementioned embodiments, as other values can be adopted. Suchvariations are not to be regarded as a departure from the spirit andscope of the invention. Rather, the scope of the invention shall bedefined as set forth in the following claims and their legalequivalents. All such modifications as would be obvious to one skilledin the art are intended to be included within the scope of the followingclaims.

1. A zoom optical system having an object side and an image side andcomprising, arranged along an optical axis in order from the object sideas follows: a first lens group having positive refractive power thatincludes a prism for bending the optical axis and at least one lenselement having positive refractive power; a second lens group havingnegative refractive power; and a third lens group having positiverefractive power; wherein said second lens group and said third lensgroup move along the optical axis during zooming and said third lensgroup moves along the optical axis toward the object side and thentoward the image side during zooming from the wide-angle end to thetelephoto end.
 2. The zoom optical system of claim 1, wherein the zoomoptical system includes an image surface on the image side and thefollowing condition is satisfied:0<( Lm−Lt)/Fw<0.2 where Lm is the distance along the optical axis fromthe vertex of the most object-side lens surface of the third lens groupto the image surface when the third lens group is nearest the objectside during zooming; Lt is the distance along the optical axis from thevertex of the most object-side lens surface of the third lens group tothe image surface at the telephoto end of the zoom range; and Fw is thefocal length of the zoom optical system at the wide-angle end of thezoom range.
 3. The zoom optical system of claim 1, wherein said thirdlens group includes a stop for controlling the amount of light thatpasses through the zoom optical system.
 4. The zoom optical system ofclaim 2, wherein said third lens group includes a stop for controllingthe amount of light that passes through the zoom optical system.
 5. Thezoom optical system of claim 1, wherein said first lens group includes,arranged along the optical axis in order from the object side asfollows: a lens element having negative refractive power; a prism; andsaid lens element having positive refractive power.
 6. The zoom opticalsystem of claim 2, wherein said first lens group includes, arrangedalong the optical axis in order from the object side as follows: a lenselement having negative refractive power; a prism; and said lens elementhaving positive refractive power.
 7. The zoom optical system of claim 3,wherein said first lens group includes, arranged along the optical axisin order from the object side as follows: a lens element having negativerefractive power; a prism; and said lens element having positiverefractive power.
 8. The zoom optical system of claim 5, wherein saidlens element having positive refractive power includes at least oneaspheric surface.
 9. The zoom optical system of claim 6, wherein saidlens element having positive refractive power includes at least oneaspheric surface.
 10. The zoom optical system of claim 7, wherein saidlens element having positive refractive power includes at least oneaspheric surface.
 11. The zoom optical system of claim 1, wherein saidthird lens group comprises: a first lens component that includes a lenselement having positive refractive power that is cemented on its imageside to a lens element having negative refractive power; and a secondlens component that is a lens element and that is on the image side ofsaid first lens component.
 12. The zoom optical system of claim 2,wherein said third lens group comprises: a first lens component thatincludes a lens element having positive refractive power that iscemented on its image side to a lens element having negative refractivepower; and a second lens component that is a lens element and that is onthe image side of said first lens component.
 13. The zoom optical systemof claim 3, wherein said third lens group comprises: a first lenscomponent that includes a lens element having positive refractive powerthat is cemented on its image side to a lens element having negativerefractive power; and a second lens component that is a lens element andthat is on the image side of said first lens component.
 14. The zoomoptical system of claim 5, wherein said third lens group comprises: afirst lens component that includes a lens element having positiverefractive power that is cemented on its image side to a lens elementhaving negative refractive power; and a second lens component that is alens element and that is on the image side of said first lens component.15. The zoom optical system of claim 8, wherein said third lens groupcomprises: a first lens component that includes a lens element havingpositive refractive power that is cemented on its image side to a lenselement having negative refractive power; and a second lens componentthat is a lens element and that is on the image side of said first lenscomponent.
 16. The zoom optical system of claim 11, wherein at least onesurface of said second lens component is an aspheric surface.
 17. Thezoom optical system of claim 12, wherein at least one surface of saidsecond lens component is an aspheric surface.
 18. The zoom opticalsystem of claim 16, wherein said second lens component is made ofplastic.
 19. The zoom optical system of claim 17, wherein said secondlens component is made of plastic.